Neurological exams to determine neuro-muscular function, which is typically compromised in patients with inherited mitochondrial diseases. Standard clinical neurological/neuromuscular assessment scales will be used

Standard laboratory tests to evaluate organ function will be used to assess adverse effects on organ systems and function. Electrocardiograms will be recorded to assess any effect on cardiac conduction. Subjects will be monitored for any clinical adverse signs at least monthly, and more frequently if patient condition warrants. In each case of reported adverse events, an assessment will be made if the event is due to EPI-743 administration or to underlying/intercurrent disease.

Change in Newcastle Pediatric Mitochondrial Disease Score from baseline at 13 weeks [ Time Frame: At baseline and at 13 weeks ] [ Designated as safety issue: No ]

The Newcastle Pediatric Mitochondrial Disease Score (NPMDS)is a validated scale to assess the clinical severity of mitochondrial disease. The NPMDS will be scored at baseline and at 13 weeks, and the difference will be assessed as improved, stable or deteriorated.

Secondary Outcome Measures:

Pharmacokinetics of EPI-743 after first dose and at steady state [ Time Frame: At the beginning of the study (baseline) and after 4 weeks of treatment ] [ Designated as safety issue: No ]

Serial blood samples (4-8 samples, 1.5 mL)will be drawn after the first dose of 50 mg and after the first dose escalation to 100 mg. Plasma concentrations ofEPI-743 will be analyzed and pharmacokinetic parameters calculated.

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Please refer to this study by its ClinicalTrials.gov identifier: NCT01370447